Optical: systems and elements – Optical amplifier – Raman or brillouin process
Reexamination Certificate
2000-08-11
2004-09-21
Black, Thomas G. (Department: 3663)
Optical: systems and elements
Optical amplifier
Raman or brillouin process
C359S337000
Reexamination Certificate
active
06795235
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical transmission line enabling Raman amplification of an optical signal when pumping light is supplied thereto, a method of making this optical transmission line, and an optical transmission system using this optical transmission line.
2. Related Background Art
An optical fiber amplifier amplifies optical signals so as to compensate for their loss when they propagate through an optical transmission line in an optical communication system. This optical fiber amplifier comprises a light-amplifying optical fiber and pumping light supply means. Namely, when pumping light of a predetermined wavelength is supplied from the pumping light supply means to the light-amplifying optical fiber, and an optical signal is inputted to the light-amplifying optical fiber, thus inputted optical signal is amplified by the light-amplifying optical fiber, and the amplified signal is outputted therefrom.
Known as such optical fiber amplifiers are one in which an optical fiber whose optical waveguide region is doped with a rare-earth element (e.g., Er element) is used as the light-amplifying optical fiber (hereinafter referred to as “rare-earth element doped optical fiber amplifier”) and one in which Raman amplification is utilized (hereinafter referred to as “Raman amplifier”). While the rare-earth element doped optical fiber amplifier is disposed in a repeater or the like as being formed into a module, which is only used as a discrete amplifier, the Raman amplifier can not only be used as a repeater but also amplify optical signals in an optical transmission line (optical fiber) through which the optical signals propagate, which is so-called distributed amplifier. Therefore, if Raman amplification is utilized, then not only the effective loss in the optical transmission line can be reduced, but also optical Kerr effects can be restrained from occurring due to the fact that the power of optical signals at each location in the optical transmission line becomes too high.
For example, Japanese Patent Publication No. 2617612 discloses a technique using an Er element doped optical fiber together with Raman amplification, so as to attain a uniform power distribution of optical signals in the longitudinal direction of the optical fiber. On the other hand, reference 1—L. F. Mollenauer, et al., IEEE J. of Quantum Electron., Vol. QE-22, No. 1, pp. 157-173 (1986)—describes Raman amplification caused by bidirectional pumping, thus disclosing a technique for lowering the effective loss in optical transmission lines. Also, reference 2—H. Masuda, et al., EOC′99, II-146 (1999), reference 3—H. Suzuki, et al., ECO′99, PD2-4 (1999), and reference 4—T. N. Nielsen, et al., ECO′99, PD2-2 (1999) disclose techniques for Raman amplification of optical signals by use of a dispersion-shifted optical fiber or non-zero dispersion shifted fiber as an optical transmission line. While U.S. Pat. No. 5,778,128 discloses the hybrid transmission line that is composed of dispersion compensated fiber inserted between two single mode optical fibers with equal length.
SUMMARY OF THE INVENTION
The inventors have studied the conventional techniques mentioned above and, as a result, have found problems as follows. Namely, while the Raman amplification technique disclosed in Japanese Patent Publication No. 2617612 also employs an optical amplification technique using an Er element doped optical fiber, so as to attain a uniform power distribution of optical signals in the longitudinal direction of the optical fiber, the optical transmission line is not optimally designed for the case where Raman amplification is used alone. Also, this Raman amplification technique necessitates a pumping light source for supplying pumping light for pumping Er element. If the supply of pumping light is stopped due to a failure of the pumping light source and the like, then the effective loss in the optical transmission line becomes so large that optical signals cannot propagate therethrough. Also, the optimal design of optical transmission lines is insufficient for making less nonlinearity and maintaining high SN ratio in the respective Raman amplification techniques disclosed in references 1 to 4. In U.S. Pat. No. 5,778,128, there is no consideration for Raman amplification and no suggestion of transmission lines suitable for distributed Raman amplification.
In order to overcome the problems mentioned above, it is an object of the present invention to provide an optical transmission line for Raman amplification which is designed more appropriately, a method of making this optical transmission line, and an optical transmission system using this optical transmission line.
In one aspect, the optical transmission line in accordance with the present invention is an optical transmission line enabling Raman amplification of an optical signal when pumping light is supplied thereto, wherein a region yielding a maximum value of a Raman gain coefficient is separated from an end portion where the pumping light is supplied by a predetermined distance along a direction in which the pumping light advances.
In another aspect, the optical transmission line in accordance with the present invention is an optical transmission line enabling Raman amplification of an optical signal when pumping light is supplied thereto, wherein a region yielding a minimum value of transmission loss at a wavelength of the pumping light is separated from an end portion where the pumping light is supplied by a predetermined distance along a direction in which the pumping light advances.
In still another aspect, the optical transmission line in accordance with the present invention is an optical transmission line enabling Raman amplification of an optical signal when pumping light is supplied thereto, wherein a region yielding a maximum value of a Raman efficiency coefficient which is a ratio of a Raman gain coefficient to an effective area is separated from an end portion where the pumping light is supplied by a predetermined distance along a direction in which the pumping light advances.
In still another aspect, the optical transmission line in accordance with the present invention is an optical transmission line enabling Raman amplification of an optical signal when pumping light is supplied thereto, wherein a region yielding a minimum value of effective area is separated from an end portion where the pumping light is supplied by a predetermined distance along a direction in which the pumping light advances.
In such an optical transmission line, the Raman amplification effect is smaller in the region where the pumping light has a higher power including an end portion to which the pumping light is supplied, and is greater in the region, separated by a predetermined distance from the end portion, where the pumping light has a lower power. As a consequence, at any point of the optical transmission line, the power of optical signal can be restrained from increasing to such an extent that optical Kerr effects occur remarkably and from decreasing to such an extent that the SN ratio deteriorates greatly, and the power of optical signal can fully be secured at the end point of optical transmission line. Further, the effective loss in the optical transmission line can be reduced.
Preferably, in the optical transmission line in accordance with the present invention, the maximum value of Raman gain coefficient is greater by at least 20% than the Raman gain coefficient at the end portion where the pumping light is supplied, the minimum value of transmission loss is smaller by at least 20% than the transmission loss at the end portion where the pumping light is supplied, the maximum value of Raman efficiency coefficient is greater by at least 20% than the Raman efficiency coefficient at the end portion where the pumping light is supplied, or the minimum value of effective area is smaller by at least 20% than the effective area at the end portion where the pumping light is supplied. Letting &agr;
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Nishimura Masayuki
Okuno Toshiaki
Black Thomas G.
Cunningham Stephen
McDermott & Will & Emery
Sumitomo Electric Industries Ltd.
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